LINQ Conditional Group

Is it possible to write a LINQ statement with a conditional group clause? Here is basically what I'm trying to do:

bool someFlag = false;

var result = from t in tableName
   group t by new { (someFlag ? 0 : t.FieldA), t.FieldB } into g
   select g;

So basically if someFlag is set to true, I want to group only by FieldB, but if it's false I want to group by FieldA and FieldB.